A characteristic of a thermopile detector is that thermal gradients or transients in its external housing, noticeable especially in small analyzers with small thermal mass, will cause an offset error in the detector signal, which degrades measurement accuracy. The thermopile is a very sensitive detector containing a plurality of thermocouple junctions. In a typical analyzer it has been measured that the signal change corresponding to the absorption caused by 0,1% by volume of CO2 in a sample gas is about 2 μV. The temperature difference in the thermopile detector would then be only about 0.13 mK. It is therefore easy to understand that even small temperature gradients in the thermopile housing may cause considerable measurement errors.
In general, thermal gradients can be divided into static state thermal gradients and dynamic state thermal gradients. Static state thermal gradients appear in constant conditions when the heat flow from the ambient into the analyzer is constant and the heat flow inside the analyzer is constant. Static state thermal gradient appears as temperature difference between any two points in the analyzer and when it appears over the thermopile detector it causes an offset error into the detector signal. If all the non-idealities of the thermopile detector can be ignored the offset error in the detector signal caused by the static state thermal gradient remains constant over the time as the system state remain constant. The dynamic state thermal gradient or transient is a gradient that changes in time as the ambient temperature changes or the internal temperature of the analyzer changes. When the ambient temperature or the internal temperature of the analyzer changes the thermal flow in the analyzer causes a variable temperature difference between any two points in the analyzer and when it appears over the thermopile detector it causes a variable offset error into the detector signal. Such errors occur with a change of the external housing temperature after, e.g., a cold start-up of the analyzer or due to a change in the ambient temperature. The amplitude of the error caused by the dynamic state thermal gradient is proportional to the temperature difference over the thermopile detector, which in turn is proportional to the rate of temperature change. The amplitude of the error decreases as the rate of temperature change decreases and as the dynamic state thermal gradient approaches the static state thermal gradient the error caused by dynamic state thermal gradient finally becomes zero and the error caused by the static state thermal gradient remains.
The U.S. Pat. No. 6,694,800 a gas analyzer is described where a beam of collimated electromagnetic radiation is provided by a fixed source, and is directed to pass said measuring volume to meet thermopile detector(s) generating an output signal indicative of a property of said at least one gas component of said mixture in the measuring volume. The thermal offset and the drift are eliminated in this analyzer by minimizing the thermal gradients over the complete detector housing, including its electrical connections. Electrical wires are composed of materials and have dimensions producing an overall thermal conductance substantially lower than that of said electrical pins. The electrical wires are connected with the electrical contact pins either directly or indirectly, and enclosed in the thermal mass together with said detector housing(s), and the electrical wires extend from the cavity through the thermal mass to the outside thereof with at least one exit point at said outer surface. Theoretically there should not be any signal offset in a thermopile without radiation reaching its sensitive area and in order to achieve this there should not be any temperature difference between the hot junctions in the sensitive area and the cold reference junctions of the thermopile. This further means that no thermal gradient can be allowed within the detector housing in spite of the relatively high heat flow and small thermal mass of the small sized analyzer. There will always be a gradient from the analyzer to the ambient but according to the invention this gradient is transferred away from the detector housing and its electrical connections by completely enclosing the detector housing in a material with good thermal conductivity.
The U.S. Pat. No. 6,694,800 thus introduces a solution for static state temperature compensation, which means that ambient temperature or internal temperature flows of the analyzer remain constant. As the detector output voltage is proportional to the temperature difference between the active and the reference junctions of the thermopile detector, it is easy to understand detectors sensitiveness to static temperature flows or static state temperature gradients. Therefore it is easy to understand also that variable temperature flows or temperature transients, in other words dynamic state temperature gradients should be eliminated also.
The U.S. Pat. No. 5,542,285 describes a method and apparatus for compensating transient errors in gas analyzer equipment caused by ambient temperature changes. A characteristic of the thermopile is that with a change of its external housing temperature after, e.g., a cold start-up of the analyzer or due to an ambient temperature change the detector output will contain a transient error which degrades measurement accuracy over the duration of the transient state. The invention is based on compensating the thermal drift of a gas analyzer by means of measuring the temperature of the thermal infrared detector, or the temperature of the detector package advantageously having the same temperature as the detector or, advantageously, the temperature of the analyzer body piece having the same temperature as the detector package, and then adding a correction signal dependent on the temperature rate of change to the detector output signal. A shortcoming of this compensation method is that the actual error source causing the temperature transient error at the detector output is not corrected, but the amplitude and the rate of change of thermal detectors or its conductive housings temperature is measured, which is thought to correspond the actual error signal, and a correction signal in certain sense proportional to measured temperature signal is finally added to the detector output signal as correction. Measuring the temperature in one or even few points of the detector case or its thermally conductive housing does not indicate the source or the direction of heat flow accurately enough to predict whether the temperature change is caused by ambient temperature change, internal thermal transient caused by e.g. a cold start-up of the analyzer or e.g. a fan cooling the one side of the sensor. Adding multiple temperature sensors in the gas analyzer body makes the analyzer uneconomical and functioning complicated through heavy computing.